A passive radar device is a device that uses a radio wave transmitted from an existing radio source such as a broadcasting station to detect an azimuth of a target, a distance therefrom, and the like by measuring a path length difference between a signal (direct wave) that directly arrives from the radio source and a signal (scattered wave) scattered by the target and a Doppler frequency shift of a scattered-wave signal.
Here, examples considered as the radio source include a global navigation satellite system (GNSS) in addition to a television or radio broadcasting station. A passive radar is attracting attention as a system useful for power saving and radio wave resource saving because of not emitting a radio wave itself (see, for example, Non Patent Literature 1).
However, a passive radar system has the most significant object of stretching a detection range. The reason that it is particularly difficult to stretch the detection range with the passive radar system is because power of the radio wave transmitted from the radio source is feeble compared to a detection radar such as a conventional active system, which extremely lowers a signal to noise ratio (SNR). In other words, in the passive radar system, in order to stretch the detection range, it is necessary to improve the SNR.
Therefore, in order to stretch the detection range, which is the most significant object of the passive radar system, there is proposed a method of observing a target for as long a period as possible and extending an integration time of a signal, to thereby increase a Doppler frequency resolution and improve the SNR (see, for example, Patent Literatures 1 to 3). However, by extending the integration time, the target moves between range cells during an observing time, which causes a problem in that there is a change in a distance from a target signal and a Doppler frequency thereof, but Patent Literatures 1 to 3 each disclose a countermeasure against this problem.
Further, in the passive radar system, in order to stretch the detection range, there is proposed a method of applying processing called “stretch processing” to a direct wave signal in order to generate a reference signal into which a movement between the range cells (range migration) corresponding to a speed of the target is incorporated in advance from the direct-wave signal and obtaining a cross-correlation between the reference signal and the scattered-wave signal, to thereby correct the movement between the range cells and extend a time that can be integrated (see, for example, Non Patent Literatures 2 and 3).